1. Field of the Invention
The present invention relates to four region semiconductor switching devices and more particularly to fast switching thyristors.
2. Description of the Prior Art
Four region semiconductor switching devices, known as thyristors, have previously been constructed with shorted emitters. It is known to construct a thyristor with a semiconducting body having at least one external emitter zone and an adjacent base zone which is electrically connected with an emitter electrode associated with the emitter zone via a plurality of shorts located in the emitter zone.
The purpose of the above-mentioned shorts is to improve the dv/dt behavior of the thyristor. The improvement in the dv/dt behavior occurs since part of the electrons flowing to the emitter zone, on the application of a voltage to the thyristor, flow directly to the emitter electrode via the shorts without giving rise to an emission of charge carriers from the emitter zone. On firing, or triggering, with an applied voltage, an emission of charge carriers from the emitter zone sets in at higher dv/dt values in the case of thyristors with a shorted emitter area, than in thyristors with a small shorted emitter area. At the same time, there is an increase in the value of the thyristor current at which the thyristor is switched into to the conducting state without a fed-in control current. This current is termed the "break-over current" i .sup.(bo).
In order to improve the dv/dt behavior or to increase the value of the current i.sup.(bo), the shorted area can be increased, for example, by increasing the diameter of the short. However, the shorts cause a hindrance for the propagation of the firing process emanating from the emitter edge adjacent the control electrode. As a result, the spread of the firing process is strongly restrained by increasing the shorted area. In addition, the voltage drop in the forward (conducting) direction of the thyristor would become relatively high on account of the relatively small igniting area.
The basic task of the invention is to improve a thyristor of the above-mentioned type in such a way that the dv/dt behavior is improved, the current i.sup.(bo) is increased, and the turn-off time is reduced without raising the forward voltage or appreciably impairing the spread of the firing process.
The present invention is based on the fact that these objectives can be achieved via a scaling down of the shorted emitter, that is, by a size reduction and an increase in the shorts at a constant ratio of short diameter to the distance of the shorts from each other.
The invention is characterized by the fact that the shorts have a diameter "d" of &lt;20 .mu.m, and by the fact that the ratio of the distance "a" between two adjacent shorts to their respective diameter is &gt;3.
A thyristor has already been described in the prior art in which the diameter of the shorts lies between 5 and 15 .mu.m. However, nothing is said concerning the distance between two neighboring shorts. A thyristor has also been described for which not only is the diameter of the shorts given, but also their spacing. In this case, however, the diameters of the shorts lie between 100 and 200 .mu.m and the distance between them is about 1 mm.